This invention relates to telephone call signaling, and more particularly, to the extension of tone-signaling service through switching offices originally designed to receive call-signaling information in the form of conventional or rotary dial pulsing.
The introduction of the tone-signaling telephone set has proceeded apace since it was made possible by various of L. A. Meacham's inventions, including those disclosed in U.S. Pat. No. 2,824,173 issued Feb. 18, 1958, entitled, "Transistor Selective Ringing, Dialing and Party Indentification Circuit," and U.S. Pat. No. 3,060,275 issued Oct. 23, 1962, entitled, "Telephone System Signaling." Numerous pre-existing telephone switching systems have been modified to serve tone-signaling telephone sets by the inclusion of conversion apparatus to receive and convert the tone signals to a form usable by the originally installed dial pulse responsive equipment. As electronic switching systems were introduced, provisions were made in their original design to serve both tone-signaling and dial-pulsing telephone sets. The goal of modifying all of the originating registers in existing No. 5 Crossbar offices, as well as the subscriber sender in existing No. 1 Crossbar offices, has, however, become an expensive undertaking. Accordingly, interim conversion equipment has been employed in some installations to avoid the necessity of wholesale replacement of registers or extensive modification thereof.
One form of conversion equipment utilizes a tone receiver that is bridged across the line. When the receiver detects the presence of a tone signal, it splits the line, provides the tone-originating (calling) end with resistance battery and ground, and repeats a dial pulse translation of the tone signal to the line relay in the register. At the conclusion of outpulsing to the register, the adapter unit removes its resistance battery and ground and cuts through the calling end to the register, which then provides the normal battery and ground conditions to the line. In effect, the conversion unit operates the register's line relay in the same fashion as would the prior art rotary dial telephone set.
Some telephone customers have experienced annoyance with the loudness of the clicks occasioned by the operation of the line-splitting relay in the conversion unit. This difficulty has been traced to the difference in impedance presented to the line when battery and ground is provided through the conversion unit on the one hand, and from the register on the other. Other customers have complained about the delay in receiving second dial after dialing an initial directing digit (usually "9") in certain private branch exchanges. This delay is attributable to the delays attendant upon the converter unit splitting and restoring the line continuity.